In judging an attack of a dental disease or caries degree in the dental clinic or from oral hygiene, various methods have been reported for examining the activity and number of germs of microorganisms such as Streptococcus mutans (hereinafter referred to as S. mutans) and Lactobacillus in saliva in the mouth or dental plaque on the teeth. Previously, a rapid method of the diagnosis has been based on the fact that microorganisms cause change in color of a specified reagent (see the specification of Japanese Pat. No. 539011 (Japanese patent application publication No. 19817/1968) in which the tests are made on general microorganisms). The method of this patent comprises, in summary, adding a given amount of (a) resazurin, (b) a neotetrazolium chloride or (c) triphenyltetrazolium chloride to a culture liquid (nutrient source for the propagation of microorganisms); immersing the test paper in the liquid; drying the test paper; dividing the test paper into small test pieces; dropping a given amount of a germ suspension to be tested on the test piece; culturing the same at 37.degree. C. for a given time; and judging the activity of the germs in the germ suspension from a change in color tone.
Various methods of determining caries activity have also been reported. A recent report of Tsutomu Shimono et al. ("The Japanese Journal of Pedodontics" 18, (3), 606-611 (1980) may be summarized as follows: dental plaque in the mouth is suspended in a physiological saline solution. A given volume (0.1 ml) of the suspension is used as a sample. Separately, a solution containing 10% of sucrose, 0.006% of bromthymol blue (B.T.B.) and 0.05% of sodium azide (Na.sub.3 N) and having pH 7.2 is prepared. The sample is inoculated to the solution and the mixture is allowed to stand at 37.degree. C. for 30 min. The color tone of the solution is changed from yellowish green (negative) to light yellow green (false-positive) and then to yellow (positive). It is described therein that as compared with the results of conventional methods of culture tests carried out simultaneously with the above-mentioned Shimono's test, a high positive relationship to the degree of children's dental caries was recognized. Masakazu Nakamura et al. reported a method of diagnosis by an examination of mouths of kindergarten pupils and results thereof in "Journal of Dental Health" 30 (4) published in December, 1980. They used resazurin, methyl red, alizarin red S, lacmoid, bromocresol green or bromophenol blue as the color indicator. A given volume of a suspension of dental plaque in physiological saline solution was added to a culture liquid comprising 0.003% of the above color indicator, together with 10% of sucrose and 0.05% of Na.sub.3 N. After effecting the culture at 37.degree. C. for 30 min, the teeth-decaying activity was judged from a change in color tone of the culture liquid. They reported that the results had a high positive relationship with the rate of dental caries of the kindergarten pupils. It is further disclosed therein that the most clear change in color tone was recognized when resazurin was used. It has been recognized in various reports that resazurin is thus an effective indicator in the determination of teeth-decaying activity. These methods can be carried out easily. However, they are all wet methods wherein saliva or dental plaque is added to a small amount of a color-developing liquid containing resazurin or a pH indicator, the mixture is warmed to 37.degree. C. and a color change after 30 min is examined. Actually, these methods have not been generalized clinically. The most ordinary conventional technique of examining teeth-decaying activity comprises mixing 0.1-0.2 ml of saliva or mixing dental plaque with about 3 ml of a culture medium and culturing the same in an incubator at 37.degree. C. for 24-72 h. This is a wet method. This method has been employed throughout our country.